Travelling wave tube operating with helically moving electrostatically guided electron beam



March 28, 1967 K. POSCHL 3,311,778

TRAVELLING WAVE TUBE OPERATING WITH HELICALLY MOVING ELECTROSTATICALLYGUIDED ELECTRON BEAM Filed Oct. 24, 1963 fizz ,Ua, :05 P65 United StatesPatent 3,311,778 TRAVELLING WAVE TUBE OPERATING WITH HELICALLY MOVINGELECTROSTATICALLY GUIDED ELECTRON BEAM Klaus Piischl, Munich, Germany,assignor to Siemens &

Halske Aktiengesellschaft Berlin and Munich, a corporation of GermanyFiled Oct. 24, 1963, Ser. No. 318,708 Claims priority, applicationGermany, Oct. 25, 1962, S 82,176 2 Claims. (Cl. 315-35) The inventiondisclosed herein is concerned with a travelling wave tube, especially ahigh power tube, operating with an electron beam which is guidedelectrostatically along a helical path extending between an outer and aninner conductor, the inner conductor, which is with respect to the outerelectrode acted on by a positive direct potential, being helically woundabout the tube axis and operating as a delay line along which anelectromagnetic wave is propagated with azimuthal delay following theelectron beam motion.

There is a travelling wave tube known in which the electron beam iselectrostatically guided along a helical path, in the space between adelay line and a cylindrical inner conductor which is with respect tothe delay line acted on by a positive direct potential. The delay lineextends helically about the cylindrical conductor which iscrosssectionally circular. The electrons moving along their paths are inreciprocal action with a wave guided along the delay line. Thereciprocal action causes the electrons in known manner to give offenergy to the high frequency field of the wave. A braking action is thusexerted on the electrons, with the consequence that the electrons reachthe equilibrium paths with a smaller radius and that the spacing thereoffrom the delay line is increased. The coupling factor between theelectron beam and the wave on the line decreases thereby steadily overthe length of the discharge path. In order to avoid this disadvantage,it has been proposed, in connection with a travelling wave tube of theknown kind, operating with an electron beam which is between an innerand an outer conductor electrostatically guided along a helical path, toform the inner conductor as a delay line.

In the proposed known travelling wave tube, the velocity of theelectrons is so dimensioned that the angular velocity thereof is uponentering the helical path equal to or negligibly greater than theangular phase velocity of the wave along the delay line, which iscoupled to the electron beam. The assumption underlying thisdimensioning of the velocity was, that the electrons, upon reaching anequilibrium path with smaller radius by giving off kinetic energy to thehigh frequency field of the wave, retain their angular velocity. Thesynchronism which was originally present between the electrons and anelectromagnetic wave propagated along the delay line, should in thismanner be retained along the length of the discharge path, which isnecessary for obtaining high officiency of the tube.

It has now been found that the efiiciency of tubes of this kind is lowerthan expected. The problem and object of the present invention thereforeresidues in discovering the reason for this phenomenon and eliminatingit.

In order to achieve high efiiciency in connection with a travelling wavetube of the initially noted type,- it is according to the inventionproposed to dimension the gunning conditions for the electron beam sothat the electrons 3,311,778 Patented Mar. 28, 1967 have, upon enteringthe helical path, an angular velocity which is by a few percent smallerthan the angular phase velocity of the wave running along the delayline, which wave is coupled to the electron beam.

The invention is based upon the recognition of the surprising fact thatthe angular velocity of the electrons becomes upon transition from apath with greater radius to an equilibrium path with smaller radius,greater by the ratio of the radii. Accordingly, if synchronism betweenthe electrons and the wave running along the delay line should alreadyobtain upon entry of the electrons into the helical path, the electronswould, upon approaching the delay line due to the reciprocal action ofthe electrons with the wave, become ever faster and would thus come outof synchronism with the wave.

This drawback, which decreases the efiiciency of the tube can beovercome by applying the gunning conditions according to the invention.The proof is obtained by a mathematical investigation, which results inthe curved coordinate system of the tube, for the path of 16 electronsper high frequency period, in a system of 32 differential equations ofsecond order, which are coupled over an integral term describing thegiving off of energy. The numerical calculation has demonstrated that adegree of efliciency can be achieved, with appropriate selection of thespacing between the outer and the inner conductor as well as thepotentials on these conductors, which comes close to the efiiciency ofknown M-type tubes.

Important details of the invention will appear from the appended claimsand from the description which is rendered below with reference to theaccompanying drawing wherein:

FIGURE 1 is a side elevational view partly in section of the travellingwave tube constructed in accordance with the principles of the presentinvention; and

FIGURE 2 is a schematic end view of the travelling wave tube illustratedin FIGURE 1 and showing schematically the various structural elementsthereof.

With reference to the drawings, the travelling wave tube of the presentinvention includes a delay line (inner conductor) 1 which is helicallywound about the tube axis. Enclosing the delay line 1 is an outerconductor or electrode 2. An electron beam 3 is electrostatically guidedalong a helical path extending between the delay line 1 and the outerconductor 2 from an electron beam generating gun 4 to an electroncollector 5.

The angular velocity of the electrons is indicated by and the phasevelocity of the wave propagated along the delay line, which is coupledto the electron beam 3, is indicated by @ph. According to the presentinvention, there must be satisfied the requirement /ph l. The angularvelocity of the electrons is adjusted by means of an adjusting device 6connected to the electron beam generating gun 4.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

I claim:

1. In the operation of a travelling wave tube operating with an electronbeam which is guided electrostatically along a helical path extendingbetween an inner and an outer conductor, wherein the inner conductor,which is with respect to the outer conductor acted on by directpotential, operates as a delay line which is helically wound about thetube axis and along which an electromagnetic wave is propagated withazimuthal delay, following the motion of the electron beam, theimprovement therewith comprising means for coupling said electron beamto the electromagnetic wave travelling along said delay line and meansfor adjusting said coupling means to provide the electrons of saidelectron beam with an angular velocity which is less than the angularphase velocity of the wave propagated along the delay line, which waveis coupled to the electron beam, with the difference in such velocitiesbeing so selected that synchronism between the electrons and said Waveis achieved only in the further course of the electron beam subsequentto such entry.

2. In the operation of a travelling wave tube, as defined in claim .1wherein the electrons of said electron beam have an angular velocityupon entering the helical path of said electron beam which is related tothe phase velocity ph of the electromagnetic wave propagated along thedelay line and coupled to said electron beam so as to satisfy therequirement /ph l.

References Cited by the Examiner UNITED STATES PATENTS 11/1957 Chodorow3155.42

OTHER REFERENCES Watkins: The Helitron Oscillator, Proceedings of theIRE for October 1958, pages 1700-1705 cited.

JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, Examiner.

R. SEGAL, S. D. SCHLOSSER, Assistant Examiners.

1. IN THE OPERATION OF A TRAVELLING WAVE TUBE OPERATING WITH AN ELECTRONBEAM WHICH IS GUIDED ELECTROSTATICALLY ALONG A HELICAL PATH EXTENDINGBETWEEN AN INNER AND AN OUTER CONDUCTOR, WHEREIN THE INNER CONDUCTOR,WHICH IS WITH RESPECT TO THE OUTER CONDUCTOR ACTED ON BY DIRECTPOTENTIAL, OPERATES AS A DELAY LINE WHICH IS HELICALLY WOUND ABOUT THETUBE AXIS AND ALONG WHICH AN ELECTROMAGNETIC WAVE IS PROPAGATED WITHAZIMUTHAL DELAY, FOLLOWING THE MOTION OF THE ELECTRON BEAM, THEIMPROVEMENT THEREWITH COMPRISING MEANS FOR COUPLING SAID ELECTRON BEAMTO THE ELECTROMAGNETIC WAVE TRAVELLING ALONG SAID DELAY LINE AND